1. Field of the Invention
The present invention relates to a fuel injection valve mainly used in an engine for a vehicle.
2. Description of the Related Art
FIG. 6 is a vertical section showing the whole construction of a conventional fuel injection valve disclosed in, for example, the Japanese Patent Publication (unexamined) No. 2002-3831.
FIG. 7 is a partial enlarged view for explaining the construction of an essential part (a magnetic path portion) of the fuel injection valve shown in FIG. 6. Hatching that indicates a section is omitted in FIG. 7.
When a microcomputer of the engine sends an operation signal to a drive circuit (not shown in the drawings) of the fuel injection valve, an electric current flows through a coil 13, whereby magnetic fluxes indicated by lines of magnetic force 100 are generated in a magnetic loop formed of a stationary iron core 11, a moving iron core 22, a yoke 16, and a housing 12. Consequently, the moving iron core 22 is attracted toward the stationary iron core 11 by electromagnetic attraction stronger than spring force of a compression spring 14.
As the moving iron core 22 is attracted toward the stationary iron core 11, a valve element 21 integrated with the moving iron core also moves toward the stationary iron core 11, thus fuel injection into the engine being carried out.
In FIG. 6 or FIG. 7, reference numeral 17 designates a sleeve made of non-magnetic metal acting as a connecting member for connecting the yoke 16 and the stationary iron core 11.
This sleeve 17 is composed of a cylindrical part in which the stationary iron core 11 is fitted, and a ring part being a ring-shaped protrusion formed on the outer circumference of an end of the yoke 16 side of this cylindrical part. FIG. 7 clearly shows that the sleeve 17 is L-shaped in cross-section.
The ring part of the sleeve 17 is welded to the yoke 16 with the ring part being in contact with the yoke 16, and the cylindrical part of the sleeve 17 is welded to the stationary iron core 11 fitted in the cylindrical part.
Therefore, the stationary iron core 11 and the yoke 16 are fixed through the sleeve 17 in their positional relation.
Numeral 17a indicates a portion where the ring part of the sleeve 17 and the yoke 16 are welded together, and numeral 17b indicates a portion where the cylindrical part of the sleeve 17 and the stationary iron core 11 are welded together.
As described above, in the conventional fuel injection valve, the sleeve 17 made of non-magnetic metal is disposed between the yoke 16 and the stationary iron core 11 in order to reduce magnetic leakage between the stationary iron core 11 and the yoke 16 to a minimum. The yoke 16 and the sleeve 17 as well as the stationary iron core 11 and the sleeve 17 are joined together by welding in order to seal fuel in.
In particular, it is necessary that the valve element of the fuel injection valve for in-cylinder injection (i.e., fuel injection valve for a vehicle) responds at a high speed, and therefore it is required to minimize eddy current generated in the sleeve 17.
In such a fuel injection valve, a thickness t of the sleeve 17 is reduced to the minimum to minimize generation of eddy current.
In the conventional fuel injection valve of above construction, in the case where the sleeve 17 is thin, the welded portion 17a where the sleeve 17 and the yoke 16 are welded together is located near a magnetic path (i.e., path of the magnetic line of force 100) of the yoke 16. Therefore the portion where temperature rises due to welding spreads partly to the magnetic path of the yoke, and this portion (i.e., inside of a semi-circle indicated by the broken lines in FIG. 7) becomes a portion 16a of which magnetic characteristic is changed (hereinafter referred to as “magnetic characteristic change portion”) and in which magnetic flux density is decreased.
Electromagnetic stainless steel mainly used as a material for the yoke 16 in fuel injection valve tends to exhibit a sharp decrease in magnetic flux density when the temperature comes up to be not lower than 900° C. (for example, the magnetic flux density being 1.10 T at 900° C. comes to decrease to 1.02 T at 950° C.) as shown in FIG. 8, whereby the electromagnetic attraction generated in the moving iron core 22 also decreases.
In the case where the fuel injection valves are mass-produced, the magnetic characteristic in the magnetic characteristic changed portion varies depending on variation in welding temperature and welding position, which eventually results in variation in electromagnetic attraction generated in the moving iron core also varies.
Hence a problem exists in that injection quantity characteristics of the produced fuel injection valves vary largely between one product and another.
FIG. 9 is a graphic diagram showing variation in injection quantity characteristic of the conventional fuel injection valves. In the drawing, the axis of abscissas indicates a drive pulse width (msec) of an injection signal impressed on the fuel injection valve, and the axis of ordinates indicates a fuel injection quantity (mm3) per injection.
As shown in FIG. 9, the variation in injection quantity characteristics of the conventional fuel injection valves ranges approximately 10% between the uppermost and lowermost injection quantities.